Minecraft Minecraft lags on High End Pc

(I hope I posted dis in the right section :) If not, move it pls)

So, i just bought this nice new pc with the following specs:
Nvidia GeForce Gtx 960 2gb ram
8 gb ram
Intel i5 3.20 ghz
Windows 8.1
Now, I think all of you would expect this beast to run minecraft on 100+ fps right? Well, it does, but not for long.
First, when I enter an singleplayer game, it lags like I have bad internet connection. On a SIGLEplayer game.
Like when i wanna hit an mob it does no damage to them, they don't move etc. When I break blocks they immidiatly replace. After some time this fixes and it's ok to play. But i kinda have this pattern as FPS for the whole game:
First i get 140~ fps
then 200~ fps
then i get 500~fps
then it starts falling to 300~
back to 100~
again 200`
and after 1 or 2 minutes, it goes below 80~ fps
and it keeps falling from the averange 200~ fps, to 60~, but that's not all !
When i'm in an OPEN area, no objects to render or anything, it drops below 30 fps, sometimes reaches 15 fps, then it gets back up to 40~-50~ fps. and goes down again and so on. Never gets back to the normal fps.
Now, before someone will say that my pc is my fault, I can run Battlefield 4 on ultra settings with 70~ fps, Battlefield BC on highes setting with 80~ fps, Nfs MW everything on ultra with 60~ and a lot of games.

And this things happen when i have no texturepack or so, and no fancy graphics or anything.

After some minutes It lags again and when i want to walk it's a pattern of move stop move stop move stop. Lagging really hard, even if in F3 it shows me that I have 200 fps.

Sometimes even when i'm mining from 300-400 fps it drops below 30, even 15, AND STAYS LIKE THAT FOR THE WHOLE GAME

Can someone tell me what's happening ?


(Please tell me if I have any grammar mistakes, i'm still an noob at english :D)

 

Idk, my computer gets a straight 60 fps with shaders and no optifine
FPS doesn't change after 60
So i wouldn't really worry... But if it's below that doing normal activities, then i would recommend setting particles to minimum

 

I do have everything on minimal/off, but after some gaming minutes it dosen't matter if i'm mining, choping trees, building, no, it starts lagging like a bad internet connection and then i have -15 fps or so... and this is weird for an high end pc...

 

Allready did. 4GB allocated for it. Still dosen't work